Droplet microfluidics is an area of increasing interest for high-throughput bioanalysis. An aqueous droplet suspended in a bio-inert medium such as fluorocarbon oil can be considered a “nanoreactor,” isolated from the environment, in which an experiment can be performed on a minimal amount of biological material. The droplet architecture is ideally suited to performing measurements on single cells and eliminates the possibility of cross-contamination with other cells. The small volume of a droplet is also advantageous as it avoids excessive dilution of the bio-content of a cell. Most important, the high throughput of hundreds or even thousands of droplets per second enables meaningful statistics in single-cell studies and studies of other material contained within a droplet.
A key component in such processing is the ability to actuate the droplets with precision in both space and time. This can be accomplished by combining hydrodynamic flow for high speed transport with dielectrophoresis (DEP) for slower but precisely controlled transport along arbitrary paths. In dielectrophoresis, a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. All particles exhibit some dielectrophoretic activity in the presence of an electric field regardless of whether the particle is or is not charged. The particle need only be polarizable. The electric field polarizes the particle, and the resulting poles experience an attractive or repulsive force along the field lines, the direction depending on the orientation of the dipole. The direction of the force is dependent on field gradient rather than field direction, and so DEP occurs in alternating current (AC) as well as direct current (DC) electric fields. Because the field is non-uniform, the pole experiencing the greatest electric field will dominate over the other, and the particle will move.
Thus, dielectrophoresis can be used to transport, separate, sort, and otherwise manipulate various objects. In the prior art, such manipulations have typically been accomplished using microfluidic devices that have electrodes deposited within the channels of the device. For example, U.S. Pat. No. 6,203,683 to Austin et al. teaches a microfluidic device for trapping nucleic acids on an electrode by dielectrophoresis, thermocycling them on the electrode, and then releasing them for further processing. The device includes a microfluidic channel that has field electrodes positioned to provide a dielectrophoretic field in the channel and a single trapping electrode positioned in the channel between the field electrodes.
According to Austin et al., the device is fabricated by forming the channel and included electrodes on a surface of a substrate and then covering that surface with a coverslip. The resulting electrodes are fixed within the channel and are an integral part of the device. As a result of using this typical method of electrode formation, dielectrophoretic manipulations can take place only in the specific locations defined by the fixed electrodes, and the electrodes are discarded along with the used device. As platinum is the particularly preferred electrode material specified by Austin et al., the electrodes can add significant cost to a disposable device.
In performing dielectrophoretic manipulations, it would be desirable in many applications to have the ability to apply electric fields at arbitrary locations within a microfluidic device rather than only at predefined locations where electrodes are deposited during fabrication of the device. Further, it would be advantageous to eliminate the cost of included electrodes to be used in dielectrophoresis in a microfluidic device, thereby providing a less expensive disposable device.